Communication between a base station and a terminal is performed on a radio-frame basis (i.e., a frame-by-frame basis). For example, in Non-patent Literature 1, it is specified that communication between a base station and a terminal is performed on a subframe basis in an LTE (Long Term Evolution) system.
A base station and a terminal include a receiver inside thereof. The receiver receives data at a plurality of timings due to a timing error ttx_err in data transmission, a multipath propagation delay tprop, and so on.
FIGS. 23A and 23B are conceptual diagrams showing a process in which a receiver receives data at a plurality of receiving timings. For example, in FIG. 23A, the receiver receives a subframe #0 at a timing t0 and a timing t1. Note that the timing t1 is later than the timing t0 by a delay tdet. The delay tdet is the sum of the error ttx_err and the multipath propagation delay tprop. Similarly, the receiver receives a subframe #1 at a timing t2 and a timing t3. Note that the timing t3 is later than the timing t2 by the delay tdet.
The reviver performs a receiving process by extracting data corresponding to one subframe by using the receiving timing t0 or t2 or the timing t1 or t3 as the start of the extraction (i.e., the head of the subframe).
It should be noted that in the state shown in FIG. 23A, the following problem occurs. For example, the receiver starts the extraction of data of the subframe #0 by using the receiving timing t1 as the start of the extraction. Further, the receiver starts the extraction of data of the subframe #1 by using the receiving timing t3 as the start of the extraction. In this case, the receiver extracts the data in such a manner that the head of the subframe #1 received at the receiving timing t2 is mixed with the end of the subframe #0 received at the receiving timing t1. Therefore, the data of the subframe #0 received at the receiving timing t1 overlaps the data of the subframe #1 received at the receiving timing t2. As a result, there is a problem that interference occurs between the subframes due to this overlap and hence the communication quality deteriorates.
As an example of a way to cope with this problem, there is a technique in which a CP (Cyclic Prefix) is added in the head of a symbol included in a subframe. FIG. 23B shows a conceptual diagram in which a CP is added in the head of (i.e., in front of) a symbol included in a subframe. Note that it is assumed that the length of the CP (hereinafter referred to as a “CP length”) is larger than the transmission timing error tdet. For example, the receiver starts the extraction of data of the subframe #0, in which the CP is added, by using the receiving timing t1 as the start of the extraction (i.e., the head of the subframe #0). Further, the receiver starts the extraction of data of the subframe #1, in which the CP is added, by using the receiving timing t3 as the start of the extraction. In the state shown in FIG. 23B, since the CP is added in each of the subframes, the data of the subframe #0 received at the receiving timing t1 does not overlap the data of the subframe #1 received at the receiving timing t2.
However, when the error between the transmission timings of base stations exceeds the CP length, the above-described interference occurs between subframes. Therefore, it is necessary to synchronize the transmission timings of the base stations with each other.
Examples of an ordinary method for obtaining synchronization between base stations (i.e., for synchronizing base stations with each other) include a method using a GPS (Global Positioning System), IEEE 1558 v2, or a reference signal.
The following are disadvantages and advantages of the aforementioned method.
Firstly, when the GPS is used for synchronization between base stations, a GPS device needs to be installed in the base stations. However, the installation of the GPS device could increase the cost for the base stations. Further, there is a problem that synchronization between base stations cannot be obtained by the GPS in an environment where a radio wave from a GPS satellite cannot be received, such as an indoor environment.
Next, when IEEE 1558 v2 is used for synchronization between base stations, the synchronization between the base stations is obtained through wired backhaul. Therefore, there is a problem that when there is a delay in the backhaul, the synchronization error becomes larger.
Further, when a reference signal is used for synchronization between base stations, one of the base stations obtains the synchronization by using a reference signal transmitted from the other base station. Since no additional device is required to obtain the synchronization, the cost for the base stations is expected to be reduced. Further, there is no obstacle to obtain synchronization between base stations even when the synchronization is obtained indoors.
As one of examples of a method using a reference signal for synchronization between base stations, Non-patent Literature 2 specifies (i.e., discloses) a so-called Network Listening (NL) method (hereinafter simply referred to as “network listening”).
An inter-base-station synchronization method using network listening in related art is described hereinafter.
FIG. 24 shows an ordinary network configuration in network listening. A source BS 1 is a base station based on which synchronization is obtained and a target BS 2 is a base station which is synchronized with the source BS 1. The source BS 1 includes independent synchronization means such as a GPS and hence can obtain accurate synchronization. Further, the source BS 1 transmits a reference signal to the target BS 2 so that synchronization between these base stations can be obtained. In contrast to this, the target BS 2 does not include any independent synchronization means such as a GPS. The target BS 2 synchronizes with the source BS 1 by using the reference signal transmitted from the source BS 1.